The present invention relates to a method for measuring the dispersion characteristics of an optical fiber to a light signal transmitted therein.
Since a laser which is a light source for emitting a light signal to be transmitted through an optical fiber for communication has a widely-spread frequency spectrum and the optical fiber has dispersion characteristics, the waveform of the light signal transmitted through the optical fiber from one end thereof and received at the other end thereof has a distortion even if the proper light signal is launched into the optical fiber at the transmitter end thereof. For that reason, there is a problem that the light signal is not received well at the receiver end of the optical fiber. Therefore, it is necessary to grasp the dispersion characteristics of the optical fiber in designing an optical communication system employing the optical fiber. Various measurement methods of the dispersion characteristics have been developed.
FIG. 3 shows a block diagram of a conventional for measuring the dispersion characteristics of an optical fiber. Shown in FIG. 3 are an oscillator 1 for supplying electrical modulation signals S.sub.1, S.sub.2, . . . and S.sub.N for laser for measurement 2.sub.1, 2.sub.2, . . . and 2.sub.N and an electric reference signal S.sub.N+1 for a phase comparator 6, the laser for measurement which emit N light signals of different wavelengths in wavelength bands for measurement, an optical switch 3 for sequentially changing over the output light signals from N numbers of the laser for measurement 2.sub.1, 2.sub.2, . . . 2.sub.N, the optical fiber 4 to be measured, a light reiver 5 by which a intensity-modulated light signal coming out from the optical fiber 4 is converted into an demodulated electrical signal D, the phase comparator 6 for detecting the phase difference between the electric demodulated electrical signal D and the reference electrical signal S.sub.N+1, output signal S corresponding to the difference, and optical fiber F.sub.1, F.sub.2, . . . and F.sub.N. The output light signals from the laser for measurement 2.sub.1, 2.sub.2, . . . and 2.sub.N are subjected to intensity modulation based on the modulation frequency F.sub.0 of the oscillator 1, and are then sent to the optical fiber 4 through the other otical fibers F.sub.1, F.sub.2, . . . and F.sub.N and the optical switch 3 so that the light signals enter one after another into the optical fiber 4. The light receiver 5 sequentially receives the light signals transmited through the optical fiber 4 and converts the signals into the electrical signals S.sub.1, S.sub.2, . . . and S.sub.N which are the demodulated electrical signals D. The phase differences between the reference electrical signal S.sub.N+1 and the demodulated electrical signals D are sequentially detected by the phase comparator 6 to measure the dispersion characteristics of the optical fiber 4. However, since the light signals and the reference electrical signal S.sub.N+1 are transmitted through mutually different media to perform the measurement as shown in FIG. 3, the fluctuation in the phases of the demodulated electrical signals, D, which is caused when the otical fiber 4 to be measured expands or contracts due to the change in the temperature or the like, cannot be prevented from affecting the result of the measurement. This is a problem.
FIG. 4 shows a block diagram of another conventional method for measuring the dispersion characteristics of an optical fiber 4. In the method, an optical reference signal and measuring light signals are transmitted through the optical fiber 4. A light signal generated by a reference laser 2.sub.0 and having a wavelength of 1.3.mu. which is nearly equal to the zero-dispersion wavelength of the optical fiber 4, is modulated at a frequency f.sub.0 so that the optical reference signal is obtained. Light signals generated by laser for measurement 2.sub.1, 2.sub.2, . . . and 2.sub.N are modulated at the frequency f.sub.0 so that the measuring light signals are obtained. The measuring light signals are sequentially sent to an optical multiplexer 13 through an optical switch 3 so that each of the signals is multiplexed with the optical reference signal by the multiplexer 13, the output light signal from which is transmitted through the optical fiber 4 and received by an optical demultiplexer 7 which demultiplexes the received light signal into the optical reference signal and the measuring light signal which are converted into demodulated electrical signals Da and Db by light receivers 5a and 5b, respectively. Since the wavelength of the output light signal from the laser for reference 2.sub.0 is nearly equal to the zero-dispersion wavelength of the optical fiber 4, the signal is hardly affected by the dispersion characteristics of the fiber. The output light signals from both the laser for reference 2.sub.0 and the laser for measurement 2.sub.1, 2.sub.2, . . . and 2.sub.N are equally affected by the expansion and contraction of optical fiber 4. The demodulated electrical signals Da and Db are differentially multiplexed with each other by a phase comparator 6. As a result, the dispersion characteristics of the optical fiber 4 can be measured without being affected by the expansion and contraction of the fiber. However, since direct modulation is performed for the laser for reference 2.sub.0 and the laser for measurement 2.sub.1, 2.sub.2, . . . and 2.sub.N, a spectral spread, which is affected by the dispersion characteristics of the optical fiber 4, is caused. For that reason, it is difficult to measure dispersion characteristics of the fiber 4 with a high resolution. This is a problem. Although the wavelengths of the output light signals from the laser for measurement 2.sub.1, 2.sub.2, . . . and 2.sub.N are separately measured in advance to determine the dispersion characteristics of the optical fiber 4 on the basis of the measured wavelengths, the wavelengths change due to ambient conditions such as temperature so that it is difficult to accurately measure the wavelengths. For the reason, there is another problem that the influence of the dispersion characteristics of the optical fiber 4 cannot be accurately compensated by using a dispersion compensation circuit having a property inverse to the dispersion characteristics of the fiber in the intermediate frequency band or the baseband.